The need for good water quality is well established. Waste water treatment facilities need to meet certain water quality parameters in their discharges. Industrial water needs to meet certain minimums before it can be used. Drinking water must meet very rigid standards before it is acceptable for human consumption. And recreational waters must be of high quality for general use.
Water quality is diminished by suspended particles, higher than acceptable concentrations of nutrients, and by toxic materials. In order to effect more than symptomatic and palliative improvement in water quality, the particle matter, nutrients, and toxins must be removed. Without removal, nutrients and toxins are recycled again and again in open waters until eventually flushed from the system or buried in the sediments to the point of becoming unavailable (1).
Previous inventions have not attempted or have not been successful at removing large amounts of nutrients from quiescent water bodies with low concentrations of nutrients. Hughes U.S. Pat. No. 4,507,206) understood the role of nutrients and particularly phosphorus in the aquatic system. His patent would inactivate phosphorus to limit eutrophication in receiving waters but would require continuous additions of precipitating agent including additions to input streams and point source discharges. Resources to accomplish this in most complex watersheds would be extensive and since nutrient or particle removal is not a part of his invention, if the process were discontinued there would be reverting or even increased eutrophication of the water body within a short time. Also, the non-removal of sediments and addition of material by this patent would allow sediments to continue to accumulate and may even accelerate sediment buildup, a situation opposed to lake restoration. One time, chemical precipitation projects have been used to help with lake restoration (9) when used in conjunction with nutrient curtailing activities.
Nutrients and toxins are generally contained in particle matter either as a result of biomagnification or simple adsorption to organic and inorganic particles. The DeFraites U.S. Pat. No. 4,209,388 shows the usefulness of allowing unwanted nutrients to first become incorporated into organic organisms which are then separated from the water. His method requires a series of ponds, probably man made at great expense of land and energy, and is of questionable effectiveness when the concentration of nutrients is relatively low as it is in natural waters rather than the high concentrations of the sewage water of his invention.
The nutrients found in natural aquatic systems are very difficult to remove. One reason is the very low concentrations. In north temperate waters concentrations as low as 0.020 mg/1 phosphorus can cause water quality problems and concentrations above 0.030 mg/1 phosphorus are almost always detrimental. Also, the phosphorus is mostly contained in very small particles (2) of specific gravities near that of water. These small particles of algae, bacteria and detritus can be removed by sand filtration (3) and through settling chambers of waste water treatment plants (4) but these methods are very expensive, require extensive pumping, and are not efficient at treating large bodies of water. Aquatic weeds are sometimes harvested for nutrient removal (5) but efficiency is small (about half a kilogram per ton of weeds removed) and limited to lakes where weeds grow in accessible regions and limited to a few cuttings per year.
Several previous inventions have made attempts at removing fine particle matter from liquid suspensions. Lee's U.S. Pat. No. 4,701,260 uses a lamella separator to concentrate particle matter and remove the resulting sludge on a conveyor. Middlebeek's U.S. Pat. No. 4,202,778 shows a similar device except that it discharges its sediment accumulation through a bottom orifice and the lighter materials through an upper orifice, the entire chamber being under pressure. U.S. Pat. No. 4,948,518 by Turgay also uses separation chambers and a funnel shaped bottom to separate and remove a suspension of sewage sludge after proper contact time and agitation. These inventions require complex processes, regulated flow, continuous maintenance, a number of pumps and sludge handling devices, and considerable energy to treat small volumes of flow and are generally unsuitable for installation in existing bodies of water.
Gariel U.S. Pat. No. 2,673,451 took his invention in situ in an attempt to treat greater volumes of water at minimal hydraulic head loss. Although his apparatus may accomplish specific goals of reducing particle matter in the stream, it requires a flow of water across the settling plates and the long path of the settling material would allow most of the particles to be mineralized and their nutrients returned to the water prior to their removal.
Dunkers U.S. Pat. No. 4,298,471 continues with the in situ concept in that his invention adapts flow control of polluted inputs to the existing water body through a means of impervious curtains so that the polluted incoming water is contained. Although helpful, this device would have to be used at each major pollution source and requires pumping of all the inflow for treatment by conventional means. Also, this method would have little use where the pollution sources are diverse and uncontrollable. The permanent dredge system by Austin, U.S. Pat. No. 4,614,458, is an in situ system of removing accumulating sediment material. His system relies on stream accumulation of heavy sediment material and is principally a system of maintaining depth. Installing such an apparatus would require considerable marine construction, large volume grit pumps, would not be acceptable for smaller water bodies such as recreational ponds, and would not be efficient at retaining or removing low specific gravity and nutrient rich particle material.
The common path for phosphorus in natural lakes will illustrate the need for a new method of removing nutrients and particles from these, and other, quiescent water bodies:
Phosphorus in dissolved form enters the lake with stream flow or precipitation. Phosphorus also enters via terrestrial sources such as particle matter washed in by overland flow or blown in as dust, leaves, and debris (6). The particle matter is quickly mineralized releasing its nutrients. Regardless of the source, the newly entered phosphorus is quickly taken up by microscopic planktonic algae as the phosphorus becomes available. The algae either die and sink after their natural life span of about seven days or the algae is eaten by zooplankton. Zooplankton eat a great deal of the phytoplankton crop and pass fecal pellets of partially digested remains (10). Dead algae and zooplankton fecal pellets continually "rain down" through the water column. These microscopic particles account for the majority of phosphorus being "lost" from the upper water (7). Limnologist commonly sample the sinking fraction of particle matter in lakes using collection tubes suspended vertically in deep water (11) (12).
If these nutrient rich particles fall into the deep portions of the lake greater than six meters deep, the phosphorus they contain is lost for the season in the cold waters of the hypolimnion (8) which tends not to mix with the upper waters until fall overturn. If the particles fall in the shallow warm areas of the lake, they quickly decompose and release their phosphorus back into the water for uptake by the next generation of algae and weeds (2) (7). This cycling of phosphorus continues all summer long with cycle times of only one to two weeks, especially in shallow water bodies. The challenge is to capture these descending particles and remove them from the lake or pond on a daily basis (approximately) thus removing the accompanying phosphorus.